The subject invention relates generally to methods and apparatuses for calcining gypsum, and in particular to methods and apparatuses for continuously calcining natural gypsum, synthetic gypsum, or combinations of natural and synthetic gypsum. Calcined gypsum, more commonly known as stucco, is useful as a major ingredient of gypsum wallboard and plaster-based products. Stucco has the valuable property of being chemically reactive with water and will “set” rather quickly when the two are mixed together. It is this quick setting time that makes stucco ideal to work with in the mass production of wallboard.
Typically, wallboard consists essentially of a gypsum core sandwiched between two sheets of paper and is used as a cost-effective replacement of conventional plaster walls. To be commercially profitable, wallboard is typically manufactured by continuous high speed processes. Typically, gypsum (calcium sulfate dihydrate) predominately makes up the wallboard. Manufactures mine and transport gypsum to a board mill in order to dry it, grind it and calcine it to yield stucco (the “milling process”). Drying refers to the removal of the free water from the gypsum (water not bonded to calcium sulfate) and calcination refers to the conversion of calcium sulfate dihydrate to calcium sulfate. The reaction for the creation of stucco is characterized by the following equation:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2OThis equation shows that calcium sulfate dihydrate plus heat yields calcium sulfate hemihydrate (stucco) plus water vapor. This process is normally conducted in a flash calcination impact mill, of which there are several types known in the art. Such an impact mill simultaneously dries, grinds, and calcines the gypsum to produce stucco.
As mentioned above, calcined gypsum (stucco) has the valuable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. This setting reaction reverses the above-described stucco chemical reaction performed during the calcination step. The reaction proceeds according to the following equation:CaSO4.½H2O+1½H2O→CaSO4.2H2O+heatIn this reaction, the calcium sulfate hemihydrate is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction generally depends upon the type of calciner employed and the type of gypsum rock that is used.
In manufacturing wallboard, a “stucco slurry” is formed by mixing together calcium sulfate hemihydrate (stucco) that is produced from the milling process, an accelerator, a retarder, a starch, water, paper pulp, potash, and a water-reducing agent usually in a pin mixer. The stucco slurry is then discharged from the mixer through a chute which spreads the slurry on a moving, continuous bottom sheet of paper, which is slightly wider than the desired board width. A moving, continuous top sheet of paper is placed on the slurry and the bottom paper sheet so that the slurry is positioned in between the top and bottom sheets of paper to form a “wet wallboard.” Rollers with guides are used to form the wallboard to the desired thickness and width. The board then travels along rollers for several minutes, during which time the setting reaction occurs and the board stiffens. The boards are then cut into a desired length and then fed into a large, continuous oven/kiln for drying. The end product is a wallboard with a gypsum core.
In the milling process, either natural gypsum or synthetic gypsum can be used to produce the stucco. Synthetic gypsum, also known as by-product gypsum, is typically produced as a by-product in various chemical processes. For example, flue gas desulfurization (FGD) gypsum is a by-product of stack gas scrubbing processes and titanogypsum is produced by neutralizing waste sulfuric acid from the sulfate process used to manufacture titanium oxide pigment. Other examples, including but not limited to, phosphogypsum, fluorogypsum, citrogypsum and borogypsum are well known in the art. Synthetic gypsum typically includes more free water (water not bound to calcium sulfate) than natural gypsum. Natural gypsum typically includes about 1%-3% free water (unless otherwise indicated, percentages herein indicate percent by weight of the composition), compared to synthetic gypsum which is typically mechanically de-watered to contain about 6%-12% free water.
Over time, improvements have been made to the manufacturing process of wallboard that has increased the manufacturing board line speeds to an extent that the milling process cannot provide the board line with enough stucco to continuously produce the wallboard. The inability of the milling process to meet the demand is compounded by the increased use of gypsum that contains over 10% free water to create stucco. As stated above, stucco is produced by drying, grinding and calcining gypsum. In order to obtain the heat required to remove the additional free water from gypsum, the mill inlet temperatures had to be raised. The raised inlet temperatures has led to a reduction of the overall tonnage of gypsum the impact mill can process into stucco. Thus, the overall amount of stucco being manufactured was substantially reduced. This reduction in manufactured stucco coupled with the increase in demand from the board line led to a substantial amount of downtime. Manufacturers need the downtime to build a stucco supply that allows for the board line to continuously manufacture the wallboard. Unfortunately, the downtime leads to increased manufacturing costs and delays the process of making wallboard.
Thus, it would be advantageous to develop a method and apparatus for calcining gypsum that allowed for increased production of stucco. Moreover, it would be advantageous to produce a method and apparatus for calcining gypsum that also helped reduce problems that have always plagued the milling process. For example, it would be advantageous to increase air flow and prevent build up in the feed path (i.e., the area where the gypsum enters into the impact mill).